The subject matter described here generally relates to wind turbine blades, and, more particularly, to wind turbine blades having improved spar caps.
A wind turbine is a machine for converting the kinetic energy in wind into mechanical energy. If the mechanical energy is used directly by the machinery, such as to pump water or to grind wheat, then the wind turbine may be referred to as a windmill. Similarly, if the mechanical energy is converted to electricity, then the machine may also be referred to as a wind generator, wind turbine or wind power plant.
Wind turbines are typically categorized according to the vertical or horizontal axis about which the blades rotate. One so-called horizontal-axis wind generator is schematically illustrated in FIG. 1. This particular configuration for a wind turbine 2 includes a tower 4 supporting a nacelle 6 enclosing a drive train 8. The blades 10 are arranged on a hub to form a “rotor” at one end of the drive train 8 outside of the nacelle 6. The rotating blades 10 drive a gearbox 12 connected to an electrical generator 14 at the other end of the drive train 8 arranged inside the nacelle 6 along with a control system 16 that receives input from an anemometer 18.
The blades 10 generate lift and capture momentum from moving air that is then imparted to a rotor as the blades spin in the “rotor plane.” Each blade is typically secured at its “root” end, and then “spans” radially “outboard” to a free, “tip” end. The distance from the tip to the root, at the opposite end of the blade, is called the “span.” The front, or “leading edge,” of the blade connects the forward-most points of the blade that first contact the air. The rear, or “trailing edge,” of the blade is where airflow that has been separated by the leading edge rejoins after passing over the suction and pressure surfaces of the blade.
A “chord line” connects the leading and trailing edges of the blade in the direction of the typical airflow across the blade. The length of the chord line is simply called “the chord.” Since many blades 10 change their chord over the span, the chord length is referred to as the “root chord,” near the root, and the “tip chord,” near the tip of the blade. The chord lines are arranged in the “chord planes” that extend through the streamlines on the corresponding pressure and suction surfaces of the blade. Multiple “shear web planes” are arranged perpendicular to the chord plane.
As illustrated in FIG. 2, the blades 10 for such wind turbines 2 are typically fabricated by building up two or more skin or “shell” portions 20 from layers of woven fabric and resin. Spar caps 22 are placed in the shell portions 20 and are combined with shear webs 24 to form a structural support member. The shear webs 24 and spar caps 22 extend at least partially spanwise along the inside of the blade 10 and are typically configured as I-shaped members. For example, the spar caps 22 may be joined to the inside of the suction and pressure surfaces of the shell 20 or they may form part of the shell. In some blades, an additional trailing edge shear web 26 may also be incorporated into the blade.
The top and bottom spar caps 22 together with the shear web 24 form the main fore-aft structural member of the wind turbine blade 10. FIG. 3 illustrates a partial view of one known spar cap and shear web. The spar cap 22 can be made from an assemblage of layers of unidirectional (UD) glass fiber tapes. Thinner root and tip sections have fewer layers of the UD glass fiber tapes. The cross-sections at any section along the length of the spar cap is are typically rectangular. Some known manufacturing methods use foam wedges 32 to bridge the gap at sections where the skin foam 34 thickness is less than the spar cap thickness. This is needed to avoid abrupt changes in the surface of the subsequent layers of glass fiber and resin. If the wedges 32 were not used, then a wrinkle or crack could appear in the subsequently applied layers. Typically, the shear web 24 is joined to the spar caps 22 using a bonding material 36, such as an adhesive. The fabrication of each of these constituent parts is in itself an involved and labor-intensive process comprised of laying out fabric, glass fibers, and foam, followed by or with intervening resin application steps. In addition, the thickness of the shear web 24, in the span-wise direction, should be thick enough to provide enough surface area to securely bond with adhesive 36. As a result, the shear web 24 is often much thicker, and heavier, than needed for structural purposes. This is a disadvantage from a weight perspective.